This invention relates to a fail-safe function for dealing with failure of a vehicle velocity sensor in an automatic transmission controlled by a microcomputer.
In a microcomputer-controlled automatic transmission, shifting generally is controlled on the basis of signals from a vehicle velocity sensor and throttle opening sensor. The automatic transmission comprises a well-known group of planetary gears and a friction engagement unit for locking and releasing the elements of the planetary gears. By selectively actuating friction elements automatically in a hydraulic circuit in dependence upon the traveling condition of the vehicle, the vehicle can be made to travel in an optimum speed. On the basis of a combination of on/off signals applied to shift solenoids 1, 2, shown in FIG. 1, to energize or de-energize them, the aforementioned friction elements are selectively actuated. More specifically, an electronic control unit (ECU) includes an input signal converting circuit 5, a CPU 6 and a solenoid driver circuit 7. Output signals from a vehicle velocity sensor 3 and a throttle opening sensor 4 are applied to the CPU 6 via the input signal converting circuit 5. The CPU 6 has a memory storing gear-change patterns. A gear-change pattern which corresponds to the shift position (e.g. D range or L range) is selected, an optimum speed commensurate with the vehicle velocity and throttle opening is decided on the basis of the selected gear-change pattern, and an appropriate signal is delivered to the solenoid driver circuit 7 to actuate the solenoids 1, 2 accordingly.
When the vehicle velocity sensor develops an abnormality in the conventional microcomputer-controlled automatic transmission described above, the signal indictive of vehicle velocity is no longer produced despite the fact that the vehicle is traveling. As a result, the microcomputer mistakenly renders a decision to the effect that the velocity is zero, i.e. that the vehicle is at rest, and places the automatic transmission in first gear. This produces a gear-change shock and impedes vehicle travel. In order to solve this problem, two vehicle velocity sensors are provided, one on the transmission output shaft and one inside the speedometer or the like, for the purpose of obtaining velocity signals in each of the two systems. Then, if one of the velocity sensors malfunctions, the signal from the other sensor will be applied to the microcomputer. In other words, the transmission is provided with a fail-safe function. However, if both of the velocity sensors should happen to malfunction simultaneously, the original problem will arise. Thus, the aforementioned solution is not an effective one.